The present invention relates generally to ferromagnetic material detectors and more particularly to ferromagnetic material detectors which operate due to changes in impedance and still more particularly which operate due to changes in inductance.
It is desirable to detect the presence or passage of ferromagnetic materials in or through a surveillance area. One example of a ferromagnetic material which is intended to be detected is the presence or passage of vehicles in a traffic roadway. It is generally desirable to detect such vehicles to control traffic actuated signals, to count the number of vehicles passing a given point in a roadway or to detect the movement of vehicles along a surveillance area on a roadway. A second example of ferromagnetic materials which may be detected are ferromagnetic objects at security checkpoints. Notable examples of this detector are airport security screens in which boarding passengers to commerical airline flights are searched for ferromagnetic objects, which may include firearms or knives or other weapons.
One means of detecting the presence or passage of ferromagnetic materials relative to a surveillance area is through the use of inductive loops. Such inductive loops are widely used in traffic control situations. Typically a loop of wire is buried in the roadway in proximity to a surveillance area. Typically this loop is approximately six feet (1.83 meters) by six feet (1.83 meters) square and contains approximately three turns. The loop thus constructed is excited with an alternating current and the loop is coupled to a resonant circuit. The presence of a ferromagnetic material, or a vehicle, causes the inductance of the loop to change. The change in inductanct in turn causes the frequency of the resonant circuit to change which may be detected with a suitable detector.
The loop so buried in a roadway near a surveillance area is utilized essentially as an "air core" inductor. Capacitors are then coupled to the loop and included as part of an oscillator to produce a resonant circuit. Small changes in the value of the loop inductance caused by the presence of vehicles will effect a slight change in the resonant frequency. The presence or passage of a vehicle in proximity to the sensing loop causes the loop inductance to decrease. Small eddy currents are generated in the mass of the vehicle by transformer coupling. These eddy currents in turn create magnetic fields of their own. The polarity of the magnetic field associated with the eddy currents is at each instant opposite to that of the inductive loop. This results in a partial cancellation of the magnetic field produced by the alternating current in the inductive loop. This partial cancellation therefore causes less energy to be stored in the loop's magnetic field and therefore the inductance of the loop decreases. In a typical single six foot (1.83 meters) square, 70 microhenry loop for example, it is typical to observe an inductance decrease of about 31/2 microhenries for automobiles, 70 nanohenries for small motorcycles, and 15 nanohenries for bicycles.
There are several problems, however, which occur due to the use of an inductive loop in a traffic roadway. The multiturn wire loop is embedded in the surface of the pavement of the lane in which surveillance is desired. The wire conductors of the loop are generally laid into a narrow slot which is saw-cut into the pavement surface. The narrow slot is then sealed with an epoxy, urethane rubber, bitumen or similar material. The sealing of the narrow slot is to prevent loop wire movement. Any translation or flexure in the loop wire can cause spurious reductions in inductance with the result of false detections of vehicles. The sealing is also necessary to block water flow to the conductors since the presence of water can also alter the effective inductance of the wire loop. Another problem of an inductive loop sensor is that it is a relatively large aperture sensing device. That is, since an approximately six foot (1.83 meters) square loop is generally used, the position resolution of the sensor cannot be less than the six foot (1.83 meters) square. This reduces a vehicle count accuracy due to the low resolution of the large sampling area. A further problem with a loop sensor is that its performance depends upon the quality of the pavement surface. Any movement of the paving surface due to fractures can cause false detections. Similarly it is generally not feasible to utilize loop sensors on roadways which are unpaved or where the paving material is discontinuous, such as in cobblestones, bricks, rock or gravel. A still further disadvantage of an inductive loop sensor is that the process of sawcutting the narrow slot into which the inductive loop sensor wires are laid does considerable damage to the pavement. This results in a shortening of the pavement life and a deterioration in the surface quality of the pavement. A still further problem with inductive loop sensors is that the life of the wires utilized in the inductive loop sensors is relatively short. Frost heave, surface erosion, snowplows, chemical attack, pavement flow, all combine to damage the wires of the inductive loop sensor and may cause it to provide intermittent or continuous opens or leaks.
There are, of course, a wide variety of loops, sensors and wire configurations which may be utilized to produce a loop sensor. One example of a wire configuration which may be used as an inductive loop sensor is illustrated in U.S. Pat. No. 3,984,764, S. J. Koerner, Inductive Loop Structure for Detecting the Presence of Vehicles Over a Roadway, which issued on Oct. 5, 1976, which is hereby incorporated by reference.
Another means of detecting the presence of ferromagnetic materials, vehicles, are magnetometers and typically flux gate magnetometers. An example of a magnetometer is described in U.S. Pat. No. 3,249,915, R. J. Koerner, Method and Apparatus for Vehicle Detection issued May 3, 1966, which is hereby incorporated by reference. A magnetometer is a device inserted in the surface of a roadway to detect changes in the flux lines of the earth's magnetic field. Normally, the earth's magnetic field produces uniformly-spaced flux lines in a given relatively small surveillance area. The presence of a ferromagnetic material, a vehicle, warps the flux lines produced by the earth's magnetic field. Since the ferromagnetic material represents a path of lower reluctance for the flux lines, the flux lines are warped with more of the flux lines passing through the cross-sectional area of the ferromagnetic material than would normally pass through the same area without the presence of the ferromagnetic material. A typical magnetometer has a primary and a secondary winding. The secondary winding is D.C. biased to provide the proper operating point in relation to the intensity and direction of the earth's magnetic field at the point on the earth's surface where the surveillance area is located. The primary winding of the magnetometer is excited with an alternating current field at a frequency F. This produces a signal on the secondary winding at a frequency 2F. The amplitude and phase of the signal present on the secondary winding of the magnetometer varies in conjunction with the presence or passage of a ferromagnetic material through the surveillance area. One example of a magnetometer of this type is described in U.S. Pat. No. 3,319,161, J. C. Beynon, Flux Gate Magnetometer Utilizing a Thin Helical Strip of Magnetic Material as its Core, which issued on May 9, 1967.
The use of magnetometers has the advantage in that the package is relatively small and compact having a diameter typically of only one or two inches and a length of only five or six inches. Thus, it is not necessary to require long saw cuts in the paving material in order to produce the narrow slots to accomodate the wire conductor of an inductive loop sensor. However, magnetometers also suffer a significant disadvantage. First, it is a precision operation and very time-consuming to adjust the bias of a particular magnetometer for its proper operation in relation to the magnitude of the earth's magnetic field and the angle of incidence of the magnetic flux lines to the magnetometer at a particular surveillance location. Secondly, the flux gate magnetometer is not a simple device; it requires two primary windings and two secondary windings with an alternating current excitation of the primary winding and a D.C. bias to the secondary winding. This more complex construction necessitates the use of a four-wire cable to connect the magnetometer to a detector while an inductive loop requires only a two-wire cable. Further, the sensitivity of magnetometers varies inversely with the number of magnetometers coupled together. Thus, if two magnetometers were coupled together, the sensitivity of the combination would be half of the sensitivity of either alone. The characteristics which magnetometers and inductive loops manifest are different and not compatible.
Thus, it would be highly desirable to develop a structure which has the approximate size and configuration of a magnetometer and which will work with existing loop detection equipment. Such a device would have the advantages of a relatively small aperture sensing area, the operation of the system would not depend upon the quality of the pavement surface, the device could be installed on unpaved roads, the quality and longevity of the pavement would not be reduced due to the necessity for making saw cuts and the longevity of the device would be increased because it would not be subject to flexing of the roadway over a large surface area. Such a device has the further advantage in that it can be used with wellknown loop detector sensors and it could be retrofitted in existing applications. The device would not require the precise adjustment techniques required of existing magnetometers.
One example of an inductive loop detector which may be utilized in conjunction with an inductive loop to detect the presence of a ferromagnetic material is described in U.S. Pat. No. 3,989,932, R. J. Koerner, Inductive Loop Vehicle Detector which issued on Nov. 2, 1976, which is hereby incorporated by reference. In general, the inductive loop detector in the '932 patent has an oscillator circuit connected to the loop sensor for oscillating the loop at a frequency depending upon the inductance of the loop. It has a timing means for measuring the time duration of a fixed number of cycles of the oscillator circuit, and a reference determinator defining a reference duration, a comparator for determining the difference between the measured time duration and the reference duration, and a threshold determinator responsive to the difference exceeding a threshold value for generating a signal indicative of the presence of a vehicle in a specified area. Since the inductive loop detector operates by measuring changes in the frequency of oscillation of a resonant circuit, a change in inductance in the loop sensor will cause a change in the frequency of oscillation allowing the inductive loop vehicle detector described in the '932 patent to operate.